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Magnetic dipoles in magnetic materials are aligned when placed under an external magnetic field. For paramagnets and ferromagnets, dipole alignment occurs in the direction of the magnetic field. However, the dipoles align opposite to the field in the case of diamagnets. This state of magnetic polarization due to the external field is called magnetization. Magnetization is defined as the dipole moment per unit volume. It plays a similar role to polarization in electrostatics.
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The number of nuclear spins aligned in the lower energy state is slightly greater than those in the higher energy state. In the presence of an external magnetic field, as the spins precess at the Larmor frequency, the excess population results in a net magnetization oriented along the z axis. When a pulse or a short burst of radio waves at the Larmor frequency is applied along the x axis, the coupling of frequencies causes resonance and flips the nuclear spins of the excess population from the...
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In the absence of an external magnetic field, nuclear spin states are degenerate and randomly oriented. When a magnetic field is applied, the spins begin to precess and orient themselves along (lower energy) or against (higher energy) the direction of the field. At equilibrium, a slight excess population of spins exists in the lower energy state. Because the direction of the magnetic field is fixed as the z-axis,  the precessing magnetic moments are randomly oriented around the z-axis.
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Adaptive multipole models of optically pumped magnetometer data.

Tim M Tierney1, Zelekha Seedat2, Kelly St Pier2

  • 1Department of Imaging Neuroscience, UCL Queen Square Institute of Neurology, University College London, London, UK.

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|March 4, 2024
PubMed
Summary
This summary is machine-generated.

Adaptive Multipole Models (AMM) offer robust interference rejection for Optically Pumped Magnetometer (OPM) data, adapting multipole expansions for diverse OPM systems and improving signal-to-noise ratio.

Keywords:
Magnetoencephalographyinterference correctionoptically pumped magnetometer

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Area of Science:

  • Neuroimaging
  • Biophysics
  • Signal Processing

Background:

  • Multipole expansions are vital in Magnetoencephalography (MEG) for interference mitigation and brain signal modeling.
  • Adapting these models for Optically Pumped Magnetometer (OPM) systems is challenging due to diverse sensor and array designs.

Purpose of the Study:

  • To adapt multipole models for stable brain signal and interference modeling across various OPM systems.
  • To introduce a novel method for robust interference rejection and signal enhancement in OPM data.

Main Methods:

  • Utilized prolate spheroidal harmonics for compact brain signal representation on the scalp surface.
  • Developed Adaptive Multipole Models (AMM) employing orthogonal projection for interference rejection.
  • Compared AMM with Signal Space Separation (SSS) regarding stability, noise, and nonlinearity error robustness.

Main Results:

  • Prolate spheroidal harmonics provided compact brain signal representation with as few as 100 channels.
  • AMM demonstrated robust interference rejection across OPM systems, even with nonlinearity errors.
  • AMM achieved up to 40 dB software shielding for visual evoked responses in a 128-channel OPM system.

Conclusions:

  • AMM offers a stable and effective method for interference rejection and signal enhancement in OPM-based neuroimaging.
  • AMM provides superior robustness to sensor nonlinearity errors compared to traditional SSS.
  • The method successfully maximized signal-to-noise ratio in real OPM data for visual evoked responses.